1. Field of the Invention
The present invention relates to a method for image formation, by which an electrostatic latent image formed on an image supporting member having a surface layer is developed with toner.
2. Description of the Prior Art
In recent years, the method for image formation, by which an electrostatic latent image formed on a photosensitive member having a surface layer is developed with toner, has been drawing attention. For example, in the electrophotography, there has been known a technique that an electrostatic latent image is formed on a photosensitive member having a carbon-based coating layer of high-hardness as a surface layer, and then developed with toner containing resin components and colorants (e.g., Japanese Patent Laid-Open Publications SHO 61-25154 and SHO 63-97962). Such a carbon-based high-hardness coating layer is so hard that many times repetition of image formation process would not cause the photosensitive member to be scraped.
However, if the image formation process is repeatedly executed by using the photosensitive member having a carbon-based high-hardness coating layer as a surface layer, such as a photosensitive member comprising a surface layer of amorphous silicon carbide and a photosensitive layer of amorphous silicon, or a photosensitive member comprising a surface layer formed of an amorphous hydrocarbon layer and a photosensitive layer formed of an organic photosensitive layer, there would arise problems of toner fusion and image flow. This problem of image flow will noticeably take place especially under high humidity environments. Since the surface layer of the photosensitive member is almost free from layer-scraping, electrical charging products, such as nitrates due to nitrogen oxides (NOx) and others generated in the charging process, in which the photosensitive member is uniformly electrically charged, will be accumulated on the surface layer while the image formation process is repeated. These charging yields cause the surface layer of the photosensitive member to lower in resistivity, such that the image flow may gradually occur particularly under high humidity environments.
The toner fusion is a phenomenon that the toner melts and adheres onto the surface of the photosensitive member. This phenomenon could be attributed to the following reasons A to C. That is, A: Carriers that have deteriorated due to long-term use adhere onto the surface of the photosensitive member and are embedded therein during the process of blade cleaning. The embedded carriers serves as nuclei and toner fuses on the photosensitive member surface during the process of blade cleaning. B: Minute projections generated in the production process of the photosensitive member serve as nuclei for toner fusion to take place. C: Because of deterioration or damage in the cleaner blade due to long-term use, abnormal stress may be applied to the toner during the process of cleaning, causing occurrence of toner fusion. Normal organic photosensitive members (OPC) undergo layer-scraping during the process of cleaning, so that fused toner is also removed at that time. On the other hand, the photosensitive member having the above mentioned hard surface layer, it is considered, does not involve layer-scraping, so that once toner fusion has occurred, the fusion grows with the subsequent use of the photosensitive member.
Also, the photosensitive member having a carbon-based high-hardness coating layer has a tendency that its residual potential will rise due to lower mobility of the coating layer. Accordingly, when image formation is repeatedly effected, fogging and toner scattering in white grounds are likely to occur.
Also, the aforementioned photosensitive member having a carbon-based high-hardness coating layer causes the cleaning blade to undergo quite great stress during the process of blade cleaning, compared with ordinary organic photosensitive members. The reason could be that while an organic photosensitive member has its surface scraped by the blade to some extent so that the stress applied to the blade is relaxed, the photosensitive member having a carbon-based high-hardness coating layer involves almost no occurrence of layer scraping. Thus, there would arise a problem that a cleaning defect of the toner slipping through the blade is likely to occur due to deformation of the blade, and another problem that the slipped-through toner accumulates on the back face of the blade, causing a remaining latent image on the photosensitive member surface to be developed.